A very popular method, for example, what has come to be known in English as “Filtered Back Projection (FBP)”. In this type of CT imaging, the examination object is penetrated—starting from an x-ray source (x-ray tube, focus)—by a, for example, conical or pyramidal beam that is captured, more or less attenuated after penetration by a detector array of planar design. The measured values are filtered in a suitable way and back projected, as a result of which at least one slice image is obtained that, owing to the different absorption values of the respective x-rays, represents the anatomy in this slice region by different grey scale values.
The so-called “weighted FBP method” has been developed in order to avoid “cone beam artifacts” produced because of the conical or pyramidal x-ray beam, and to make better use in the interest of an optimized beam dose of redundant data such as are produced during spiral scans with a low table feed owing to multiple irradiation of one and the same voxel and also to avoid redundancy artifacts.
The spiral scanning just mentioned constitutes an alternative to conventional or sequential CT imaging that cannot be wished away. The development of slip ring technology has rendered it possible for the scanning system, that is to say the tube detector array arrangement, to be continuously rotated. If the patient table is likewise subjected to continuous feed during this rotation, the x-ray source revolves around the patient on a spiral or a helix. The object to be examined can be recorded over a large volume in this way without any gaps, recording time for such a large volume being decisively shortened by comparison with conventional tomography. Recording without any gaps means: all the points along the z-axis (spiral axis) have been trans-radiated at least once. At least partial information is present for the reconstruction from each layer, which can subsequently be selected at will. Missing data of this layer can be supplemented by interpolation (if necessary, after appropriate rebinning), after which all known two- or three-dimensional reconstruction methods are available again without restriction.
DE 102005038561 A1 discloses a variable pitch with the aim of optimally recording the contrast medium by way of an x-ray CT machine.
Again, WO 2004084137 A2 discloses a variable pitch with a novel reconstruction algorithm based on interpolation.